No. 1 Field of the Invention
The invention relates to a structure of a corrugated ceramic heat transfer block for a cross flow heat exchanger.
No. 2 Description of the Prior Art
Conventionally cross flow heat exchangers utilizing corrugated ceramic heat transfer blocks have been used, wherein the heat transfer block utilized therein comprises heat transfer elements 10 each of which is made of a corrugated plate of ceramics 11 and a planar plate of ceramics 12 abutting each other, for example as shown in FIG. 4. As shown in FIG. 5, a plurality of heat transfer elements 10 are stacked in multi-layers so that the direction of corrugation of corrugated plate 11 in any element alternately intersects at a right angle that in an adjacent element, thereby forming a heat transfer block 20. Two kinds of fluids at different temperatures flow through the heat transfer block 20 in two directions shown by arrows A and B to exchange heat between each other.
FIG. 6 shows an example of a cross flow heat exchanger 30 which is constructed of a plurality of heat transfer blocks which are assembled and tightened after considering the flow rates and allowable pressure losses of fluids flowing therethrough. In this example, for instance, hot exhaust gas flows perpendicularly through the heat exchanger 30 in the direction of arrows E, E', while cold air flows hoizontally, as shown by arrows C.sub.1, C.sub.2, C.sub.3 and C.sub.4, through the heat exchanger 30, heat being exchanged therebetween.
When heat exchange is conducted between exhaust gas E and combustion air C using this cross flow heat exchanger, a portion of the exhaust gas is successively cooled to below its dew-point as a result of heat exchanger, wherein soot in the exhaust gas E is deposited on the heat transfer surface. Since soot accumulated in the course of operation lowers the amount of heat transferred, soot blowing is conducted from either the inlet or outlet of exhaust gas E, or from both the inlet and outlet of exhaust gas, to remove the deposited soot. Cold, warm or hot water, water vapor, etc. may be suitably selected as a soot blowing fluid J injected from a soot blowing pipe 21, as shown in FIG. 7, and soot removal is effected either by a separation action due to the temperature difference between the soot blowing fluid J and the soot/heat transfer surface, or by a separation action caused by an impacting force by the pressurized soot blowing fluid J which is injected from small holes and impinged upon the soot, the separation effect being generally thought to depend on the kind of fluid in the former separation action, and on the pressure and density of the fluid in the latter action.
Referring to FIG. 6, when the heat transfer blocks 20 are installed onto a casing 22, a heat transfer block holder 23 is used on each corner of the heat transfer block 20, and a packing 23a is interposed between the heat transfer block 20 and the heat transfer block holder 23.
The corrugated plate 11 and the planar plate 12 of the heat transfer element 10 shown in FIG. 4 are made of ceramic paper having a thickness of about 1 mm. If a soot blowing impact of 0.5 to 0.7 g/cm.sup.2 is repeatedly applied by injecting soot blowing fluid J as shown in FIG. 7 for soot removal, repeating this impact about 400 times may cause throughholes 11C to develop in the top end of the corrugated plate 11, air for combustion being leaked through the holes 11C, thereby rendering heat exchange impossible and impairing the function as a heat exchanger.
On the other hand, referring to FIG. 6, since only the end 20a of ceramic paper sheet contacts the packing 23a of the heat transfer block 20, a tightening force is concentrated therein so that the end of the ceramic sheet is broken or corrugation collapses, thereby impairing sealing performance. If soft packing made of bundles of glass wool, ceramic paper or the like is used, the packing 23a comes to envelop embrace the end 20a of the heat transfer block 20 of corrugated ceramics, thereby making the contacting area larger and easing the concentration of tightening force. However, at the same time, exhaust gas and air for combustion pass through the interior of the packing, consequently impairing sealing capability.